var processedCount=0; // global variable
var totalFiles = 0; // global variable

function handleFileSelect(evt) {
  var files = evt.target.files; // FileList object

  totalFiles = files.length; // important

  // files is a FileList of File objects. List some properties.
  for (var i = 0, f; f = files[i]; i++) {
    //Create new file reader
    var r = new FileReader();
    //On load call
    r.onload = (function(theFile){
        return function(){
          onLoadHandler(this,theFile);
          onLoadEndHandler();
       };
    })(f);
    r.readAsText(f);
  }
}

function onLoadEndHandler(){
  processedCount++;
  if(processedCount == totalFiles){ 
    // do whatever - this code will run after everything has been loaded and processed
  }
}